The present invention relates in general to directional couplers and more specifically to directional couplers that have minimal dimensions.
As will be more completely described herein, a directional coupler is a linear, passive, multi port network, consisting of a pair of electromagnetically coupled signal conducting xe2x80x9clinesxe2x80x9d or structures such as strip lines or transmission lines. One of the pair of lines is a xe2x80x9cmain signal linexe2x80x9d that connects an input port of the coupler to an output port. The other of the pair of lines is an xe2x80x9cauxiliary signal linexe2x80x9d that is connected to at least one measurement or utilization port. The auxiliary line is coupled to the main line through a xe2x80x9ccoupling regionxe2x80x9d where the lines are in close proximity to each other. A radio frequency (rf) signal applied to the main line induces a signal in the auxiliary line. Maximum signal coupling between the pair of coupled lines is achieved when the length of the coupling region is an odd multiple of a quarter wavelength of the signal traveling on the main line. This attribute results in the efficient operation of a coupler having a coupling region of a given length being limited to a particular bandwidth.
Accordingly a directional coupler can perform as a measurement tool that samples a small portion of the radio frequency energy traveling through the main line between a signal source and a load, for instance. This energy can travel xe2x80x9cforwardxe2x80x9d from a signal source such as a transmitter to a load such as an antenna and/or the energy can be reflected in xe2x80x9creversexe2x80x9d from the antenna to the transmitter.
There are 3-port unidirectional couplers and 4-port bi-directional couplers. The unidirectional coupler consists of a main line and an auxiliary line, which can be internally terminated in the coupler at one end with the other end providing the coupled output. It is necessary to physically reverse the unidirectional coupler to individually measure the forward and reverse signal powers one at a time. The bidirectional coupler is similar to the unidirectional coupler with the exception that both ends of the auxiliary line provide coupled outputs. Thus the bi-directional coupler can be used for simultaneously monitoring both the forward and the reflected power.
Forward transmitter power may be monitored to determine transmitter output power and efficiency. Reflected transmitter power may be monitored to determine the state of the output transmission cable and the associated antenna. The radio communication system performance is proportional to the antenna efficiency. Comparison of the forward and the reflected powers provides a metric of communication system performance. xe2x80x9cTransmission Efficiencyxe2x80x9d, which is proportional to the ratio of the power coupled out in the forward direction to the power reflected back in the reverse direction, is dependent on the magnitude of the impedances of the electrical loads at the ports of the directional coupler.
Directional couplers are employed in a variety of electronic applications. There is a need to minimize the size and weight of such couplers which are permanently mounted in avionics or portable equipment, for example. Prior art parallel strip line couplers are sometimes laid out on printed wiring boards having straight, closely spaced conductive traces utilizing long parallel lengths to provide the coupling region. As mentioned the physical size of such couplers is a function of the wavelength of the coupled signal. These strip line couplers are useful for some applications but tend to be too long for permanent installation in avionics and portable products because of the length of the coupling regions thereof.
Accordingly other prior art directional couplers have been developed that require careful hand placement of delicate, vendor-supplied, wire wound components, which provide shortened coupling regions. Such couplers have been permanently installed in avionics equipment. A traditional engineering mandate is to reduce the number of such components requiring manual assembly.
Still other prior art couplers include main and auxiliary spiral windings in a face-to-face, mirror image planar relationship with each other. Such structures tend to result in an undesirable amount of capacitive coupling between the windings, which causes the amount of coupling to undesirably increase with frequency. It is desired for the amount of coupling to remain as constant as possible over the bandwidth of operation. Moreover such prior art structures are required to have undesirably large dimensions to facilitate electrical connection of conductive traces to the ends of the windings. Furthermore such structures can tend to allow parasitic coupling between the traces which also tends to undesirably distort the coupling characteristic over the bandwidth of operation.
Accordingly there is a need for economical directional coupler structures, which have minimal space and weight requirements that are suitable for permanent installation in aviation and portable communication systems. Also it is desirable for such couplers to provide minimal insertion losses and maximum coupling efficiencies. Additionally it is desired to provide couplers which have a constant coupling sensitivity over the bandwidth of operation and which minimize parasitic coupling. Moreover it is desirable to provide ruggedized, reliable coupler structures which don""t require hand placed or vendor supplied parts and which are easy to manufacture.